As optical fiber and optical-wireless communication network advance to accommodate fifth generation wireless networks, dual-polarization coherent optical communication schemes are being widely envisioned for use in trunk networks and deep wavelength division multiplexing (DWDM) networks to increase spectral and power efficiency ([Roberts 2009]). Greater control over phase coherence is advantageous in wireless networks that carry radio frequency services from a central office to a radio transmitter antenna by utilizing a local optical fiber network within a macro cell. By harnessing dual polarization as an orthogonal modulation scheme, all degrees of freedom of the light wave, i.e, amplitude, phase, and polarization, can be utilized simultaneously for the conveyance of data ([Li 2009], [Nakazawa 2010]).
Phase coded information cannot be detected directly by a photodetector. A CW light source having a narrow optical spectrum is required to act as an optical local oscillator such that when mixed with the data bearing optical signal on balanced photo-detectors, the amplitude and phase of the coded signal is retrieved by virtue of the linear response of the photo-detector to the incident fields and by using digital processing techniques. The phase coded information is down-converted from the optical domain to the electrical domain by virtue of the interference beating of electric fields from both the signal light and an optical local oscillator light on the photo-detector. This process is commonly referred to as “heterodyne coherent detection” ([Ip 2008]). In order to obtain reliable data down-conversion from a complex carrier wave exhibiting high order modulation, random phase fluctuations, and random polarization fluctuations in both the local oscillator and signal lights, the phase of the local oscillator light must be controlled to a high degree; better yet if the phase of the signal light were correlated to the phase of the optical local oscillator and the optical spectrum of each were to be very narrow. In general this is difficult to achieve with un-correlated or free-running optical local oscillators and signal light sources, particularly after several kilometers of optical fiber. Consequently, various ways to digitally retrieve phase information are conventionally employed. The complexity of digital data processing can be simplified if the phase of the local oscillator light were to be derived from the same light source as the signal light.
Disadvantages of some prior art approaches, including complexity and cost of optical coherent receiver equipment, will now be described. The problem of phase noise is, in part, remedied by the use of optical local oscillator sources having a narrow optical spectrum, used to down-convert baseband data from the optical domain to the electrical domain. This is followed by complex digital signal processing integrated circuits and algorithms to equalize received signals, maintain phase coherence, and partially compensate for random fluctuations. Digital data processing technology in coherent optical signal detection is described by Savory and Kuschnerov ([Savory 2010], [Kuschnerov 2009]). As shown in FIG. 1 (prior art), in the case of dual polarization coherent detection, there are four receiver channels for each polarization channel or one phase and one quadrature channel for each of two orthogonal polarizations. Therefore, two high speed, analog-to-digital converters are needed for each polarization channel, one after each analog coherent receiver, to convert the analog received signal to the digital domain. The analog-to-digital converters must then be interfaced to a digital signal processor unit which performs correction algorithms such as chromatic dispersion compensation, polarization control and equalization, carrier phase recovery, and forward error correction decoding.
To meet the demand of growing data traffic, coherent detection was introduced in Ultra-Dense Wavelength-Division Multiplexing Passive Optical Networks (UDWDM-PON), as it promotes high transmission capacity with enhanced spectral efficiency. ([Dong 2011], [Zhu 2012]). However, considering the cost, latency, and power consumption attributed to spectrally narrow optical local oscillators and digital signal processing (DSP) decoders, it may be difficult to deploy DSP-based detection in a passive optical network (PON), since in a PON architecture, the optical network units (ONUs), that convert the received optical signal to electrical signal, are located at the subscriber's premises, a location that is not under the control of the service provider. ONU environmental conditions vary and adjustments and maintenance cannot be shared with the subscriber. Thus, ONUs have to be simple, reliable and not require tuning or maintenance. Two potentially cost effective ways to render ONUs suitable for the UDWDM-PON network are: 1) replace the narrow-linewidth optical local oscillator (LO) in the ONU with a cost-effective alternative; and 2) reduce the hardware implementation complexity of the DSP unit. ([Presi 2014] and [Prat 2012]).
In prior art coherent heterodyne detection, a weak information bearing optical signal and a substantially stronger continuous wave local optical oscillator light of somewhat different but spectrally narrow optical wavelength may be mixed on a photodetector to retrieve data using received power at sum and difference frequencies, enhanced in magnitude by the stronger amplitude of the optical local oscillator. To reduce ONU cost further, a single polarization, self homodyne, optical communication link that does not use an optical local oscillator and convert data to the electrical domain by direct conversion on a photodetector has been reported. ([Shahpari 2014]). However, an external cavity laser is used at the transmitter along with additional optical filtering at the receiver. DSP and complex signal processing algorithms, for example, analog-to-digital converters (ADCs), forward error correction, static equalizer, phase recovery estimation, and dynamic equalizer, are still necessary for phase and polarization estimation. Cost reduction has consequences: slower ADCs can be used at the expense of under-sampling of the received signal, use of serial-to-parallel converters and increased filter complexity in the DSP unit. Unfortunately, only low order modulation formats with a single polarization mode have been demonstrated by using these methods.
FIG. 1 shows a prior art system 100 generally comprising a coherent optical detection scheme operating in dual polarization mode with in-phase and quadrature coding. A polarized light stream carries encoded data (see “DATA”). Orthogonal polarization components are separated by polarizer 105. A spectrally narrow external cavity laser 115 provides a local optical oscillator reference. An optical light splitter 120 divides the reference light into two paths to be combined with each of the two orthogonal polarizations by 90° optical hybrids 125. The composite light streams containing data and reference light streams are incident on pairs of balanced photo-diode detectors 130 where electric fields of each light stream are mixed by the non-linear response of the photo-diode detectors that generate a corresponding electrical waveform response representative of the in-phase and quadrature data carried by each state of polarization. The electrical response of each photo-diode detector pair is amplified by amplifiers 135 and digitally analyzed and processed by digital signal processing apparatus 110, which may perform signal correction and data recovery functions, examples of which can be: analog-to-digital conversion, channel equalization, polarization de-multiplexing, polarization mode dispersion compensation, clock recovery, phase recovery and estimation and quadrature phase shift key decoding. Much of the DSP equipment is used to compensate for channel impediments of which polarization mode dispersion, chromatic dispersion, and phase de-coherence are primary manifestations. It is desirable to reduce the magnitude of channel impediments so as to reduce the amount of DSP resources necessary to retrieve base band data.
Disadvantages of prior art means for generation of orthogonally polarized lights and information encoding thereof will now be described. Modulation formats are a key part of communication in that they enable spectrally efficient wireless and wired communication. When communicating over optical fibers, optical single sideband phase modulation has been shown to reduce unwanted chromatic dispersive effects on the light carrier wave. Optical fiber can provide long distance transportation of wireless information. Radio frequency information can be converted to the optical domain on optical sidebands of an optical carrier wavelength and transported over optical fibers and subsequently converted back to the electrical domain to propagate wirelessly. The preservation of data, specifically phase information, upon transition from optical fiber to free space electromagnetic wave propagation, requires that a coherent phase relation be maintained between the carrier frequency and the single sideband frequency. Relative phase or wavelength variations, for example spectral broadening in the optical domain, directly translate into radio frequency noise, signal fading of free-space radio waves, and loss of data integrity.
When a robust coherent phase relation exists between an optical carrier frequency having electric field oriented in a first direction and derived single or double sideband frequency or frequencies having electric field oriented in an orthogonal direction, the process of coherent heterodyne detection can be simplified without referring to an external optical local oscillator. To maintain strong phase coherence, the carrier frequency and sideband frequencies can originate from the same narrow laser source and both traverse the same optical path through various optical components, and electro-optic modulators in particular.
Further, the carrier frequency and optical sideband(s), in addition to being spectrally separated, can be orthogonally polarized relative to one another, as they propagate along the same optical channel. In this way, an isotropic channel will substantially present the same impediment mechanisms to both carrier and signal sidebands. In contrast, a crystal modulator generally presents anisotropic optical properties that depend on the polarization direction of light and direction of propagation with respect to a crystal axis of symmetry. For example, light that is polarized along a first crystal direction will be maximally modulated by a LiNbO3 electro-optic modulator while light that is polarized in an orthogonal direction will be substantially less modulated due to the intrinsic birefringence of the LiNbO3 crystal. Other electro-optic crystals such as GaAs or InP are not intrinsically birefringent and can lead to the case in which light is modulated in a first state of polarization while un-modulated in the orthogonal state of polarization. This particular property of naturally non-birefringent or isotropic crystal electro-optic modulators can be important in preventing modulation leakage between two orthogonally polarized channels.
Various prior art methods have been demonstrated that are capable of producing co-linear light streams that differ in wavelength and are orthogonally polarized relative to one another. However, these methods produce lights that are not strongly correlated in phase and therefore suffer from random noise, signal fading of free-space radio waves, and loss of data integrity. One method to produce lights having different wavelengths and orthogonal states of polarization is described by Sagues, et al. ([Sagues 2010]), which makes use of stimulated Brillouin scattering in an optically pumped optical fiber. Two parallel polarized light waves differing in wavelength are phase coherent and have spectral separation greater than the Brillouin linewidth. The Brillouin linewidth in silica glass is typically 130-210 MHz at a pump wavelength of 4880 Å. The optical fiber has low chromatic dispersion. A counter propagating pump light is polarized perpendicular to the polarization direction of two parallel polarized lights. One of the wavelength pair is chosen to fall within the Brillouin linewidth and its linear polarization gradually rotates toward the polarization direction of the pump light, while the second wavelength of the pair is chosen to lie outside the Brillouin bandwidth and its polarization remains unchanged. The technique uses an optical circulator connecting the pump light, the two parallel polarized incident wavelengths and the two orthogonally polarized exiting wavelengths, neither of which is modulated to convey information. If any one of the exiting lights were to be encoded with data, it would have to be diverted to a modulator and consequently follow a different path. In that case, its phase correlation with respect to its twin, un-modulated light, can no longer be assured.
Another prior art method that results in light streams having different wavelengths and mutually orthogonal polarizations is described by Campillo. ([Campillo 2007]). The method uses a polarization modulation crystal waveguide by means of which an initial light stream having a first wavelength and a first polarization is converted to two exiting light streams: one comprising a portion of the incident light with initial polarization, and an orthogonally polarized sideband having a second wavelength. The sideband carries no information. The introduction of an output polarizer can provide intensity on-off modulation that can be configured to convey information.
Another prior art method of producing lights having different wavelengths, at least two of which are orthogonally polarized relative to one another, is described by Zheng, et al. ([Zheng November 2014]). This technique uses a Sagnac loop interferometer, a double drive Mach-Zehnder modulator and a polarization maintaining Bragg grating optical fiber to convert an incident light stream having a first wavelength and a first polarization to an exiting light stream having the same spectral content and polarization as the incident light but having reduced intensity. An orthogonally polarized sideband is produced in the process, comprising a second wavelength. If any one of the exit light waves were to be encoded with data, it would have to be separated and consequently follow a different path. In that case its phase correlation to the un-modulated wavelength can no longer be assured.
Disadvantages of prior art approaches with LiNbO3 birefringent modulators will now be described. To date, the most common electro-optic modulator in use in telecommunication is the lithium niobate (LiNbO3), abbreviated as LN, modulator. The LiNbO3 crystal displays trigonal crystal symmetry (space group symmetry R3c) and is intrinsically birefringent with index of refraction having the uniaxial form: no=nx=ny=2.297 and no=nz=2.208. Its linear electro-optic tensor coefficients are: r13=8.6×10−12 m/V, r22=3.4×10−12 m/V, r33=30.8×10−12 M/V and r51=28.0×10−12 m/V. Since r33, along the extraordinary axis of LN, is the largest electro-optic coefficient, an electric field, Fj (j=x, y, z), applied parallel to the extraordinary axis (z-direction of the index ellipsoid) will result in the most efficient modulation. Therefore, under the external electric field: Fz≠0 and Fx=Fy=0, the index ellipsoid for LiNbO3 can be represented by:
                                                        x              2                        ⁡                          (                                                1                                      n                    o                    2                                                  +                                                      r                    13                                    ⁢                                      F                    z                                                              )                                +                                    y              2                        ⁡                          (                                                1                                      n                    o                    2                                                  +                                                      r                    13                                    ⁢                                      F                    z                                                              )                                +                                    z              2                        ⁡                          (                                                1                                      n                    o                    2                                                  +                                                      r                    33                                    ⁢                                      F                    z                                                              )                                      =        1                            Eq        .                                  ⁢        1            The z-direction is that of the extra-ordinary crystal axis in uniaxial LN.
In the case of an x-cut LN crystal, an external electric field Fz applied along the z-direction lies in the plane of the crystal surface. Prior art electrode configurations are illustrated FIG. 2, which depicts a cross section drawing of an x-cut LiNbO3 electro-optic modulator 200(a) and an electrode configuration in the case of a z-cut LiNbO3 crystal electro-optic modulator 200 (b). Features 220 and 215 represent ground (G) and signal (S) electrical contacts separated from the crystal by buffer layer 225. Electric field lines Fz are represented by features 230 and are oriented predominantly along the z-direction at the waveguide core 235, or the direction of the extraordinary axis of the crystal. TE and TM waveguide modes are both supported by the dielectric rectangular waveguides in LN. ([Wooten 2000]).
In the case of x-cut LN crystal 200(a), the optical waveguide is oriented along the y-axis (because the x-axis is vertical to the LN wafer surface and the z-axis is the direction of the applied electric field). Therefore, for light polarized with electric field along the x-axis or z-axis, the optical refractive indices are given by:nx≈ny=no−½no3r13Fz  Eq. 2nz≈ne−½ne3r33Fz  Eq. 3
Therefore, under an external modulation electrical field applied in the z-direction, the LiNbO3 crystal remains uniaxial and the optical axis remains unchanged, but the index ellipsoid is deformed by the modulation field, Fz, in accordance to Eqs. 1 and 2. Light propagating along the z-direction will experience the same phase change, independent of polarization. However, light propagating along the x- or y-direction will experience a phase change on its state of polarization. In the case of an x-cut LiNbO3 crystal, both electrodes are placed symmetrically on both sides of the waveguide such that the bias field is along the z-direction. In this case, if light is propagating along a waveguide aligned with the y-direction and is polarized along the z- (or x-) direction, then the electric field components will be modulated in accordance with Eq. 4 (or Eq. 5), where ETE, ETM, and Eo refer to the electric field amplitude of the light.ETE={circumflex over (z)}Eoe−iko(ne−1/2ne3r33Fz)y  Eq. 4ETM={circumflex over (x)}Eoe−iko(ne−1/2ne3r13Fz)y  Eq. 5Equations 4 and 5 show that for the x-cut LiNbO3 crystal modulator, the TE optical mode (polarized along the z-axis) is more efficiently modulated than the TM optical mode (polarized along the x-axis) because r33 is greater than r13 (r33/r13=3.58), resulting in a TE/TM power extinction ratio of about 20 dB, clearly contaminating the orthogonally polarized channel.
In the case of a z-cut LiNbO3 crystal modulator 200(b), an electric field applied along the z-direction means that the electric field is vertical. In this case, the waveguide can be defined along either the x- or y-direction. For example, for an optical waveguide fabricated along the y-axis, the intersection ellipse is again represented by Eq. 1, and the optical indices of refraction are the same as those in Eq. 2 and Eq. 3. However, for a z-cut crystal, the TM optical mode is polarized along the z-axis, and the TE optical mode is polarized along the x-axis. Consequently, for the z-cut LiNbO3 modulator, the TM mode is more efficiently modulated than the TE mode, by the same power ratio of about 20 dB, clearly contaminating the orthogonally polarized channel.
Conventional approaches exist for light streams containing plural wavelengths, at least two of which display electric fields that oscillate along orthogonal directions, are co-linear and correlated in phase. However, there does not exist any teaching on how to maintain phase coherence and orthogonal polarization while at the same time encode information on at least one wavelength channel in the light stream, or how to encode different information on two orthogonally polarization channels having the same wavelength as is necessary for in phase and quadrature coding.
It is with respect to these and other considerations that the various embodiments described below are presented.